Add Validation in Apex

public void beforeUpdate(List<Opportunity> newOpportunities, Map<Id, Opportunity> oldMap) {
    for (Opportunity opp : newOpportunities) {
        Opportunity oldOpp = oldMap.get(opp.Id);
        Boolean justWon = opp.StageName == 'Closed Won' && oldOpp.StageName != 'Closed Won';

        if (justWon && (opp.Amount == null || opp.Amount <= 0)) {
            opp.Amount.addError('An Opportunity cannot be marked Closed Won without a positive Amount.');
        }
    }
}

1. public void beforeUpdate(List<Opportunity> newOpportunities, Map<Id, Opportunity> oldMap) { — a new method, separate from afterUpdate.
2. for (Opportunity opp : newOpportunities) { — loops over the updated records.
3. Opportunity oldOpp = oldMap.get(opp.Id); — the prior version.
4. Boolean justWon = opp.StageName == 'Closed Won' && oldOpp.StageName != 'Closed Won'; — the same transition check as before.
5. (blank line)
6. if (justWon && (opp.Amount == null || opp.Amount <= 0)) { — only validates on the actual transition, and only when the amount is missing or non-positive.
7. opp.Amount.addError('An Opportunity cannot be marked Closed Won without a positive Amount.'); — addError(), called on the specific field, blocks the entire save and shows the message directly next to that field.
8. } — closes the if.
9. } — closes the loop.
10. } — closes the method.

addError() only works meaningfully in a before trigger — calling it in an after trigger doesn't prevent the save, since the record is already saved by the time after runs (Module 24's before/after distinction, once again). This validation needs its own beforeUpdate method on the handler, separate from the rollup logic living in afterUpdate.

trigger OpportunityTrigger on Opportunity (before update, after update) {
    OpportunityTriggerHandler handler = new OpportunityTriggerHandler();

    if (Trigger.isBefore) {
        handler.beforeUpdate(Trigger.new, Trigger.oldMap);
    }
    if (Trigger.isAfter) {
        handler.afterUpdate(Trigger.new, Trigger.oldMap);
    }
}

1. trigger OpportunityTrigger on Opportunity (before update, after update) { — this is Module 24's "Trigger Framework Fundamentals" routing pattern, now genuinely needed — one trigger, two events.
2. OpportunityTriggerHandler handler = new OpportunityTriggerHandler(); — one handler instance.
3. (blank line)
4. if (Trigger.isBefore) { — routes the before-timing event.
5. handler.beforeUpdate(Trigger.new, Trigger.oldMap); — to its own clearly-named method.
6. } — closes the if.
7. if (Trigger.isAfter) { — routes the after-timing event.
8. handler.afterUpdate(Trigger.new, Trigger.oldMap); — to its own method.
9. } — closes the if.
10. } — closes the trigger.